1. Field of the Invention
The present invention generally relates to an information recording method, an information recording device, an information reproducing method, an information reproducing device, an information recording/reproducing device, and a recording medium, and more particularly, to an information recording method, an information recording device, an information reproducing method, an information reproducing device, an information recording/reproducing device, and a recording medium which deal with data having a mark representing information of a multivalued level recorded on a cell.
2. Description of the Related Art
Conventionally, in an optical disc, binary digital data is recorded on a spiral or concentric track, in such forms as uneven pits formed by embossing, etc. (on a ROM disc), holes formed in an inorganic/organic recording film (on a recordable (write-once) disc), differences between crystal conditions (on a phase change disc), or differences between magnetization directions (on a magneto-optical disc). Upon reproducing these recorded data, a laser beam is projected on the track, and differences in intensities of reflected lights therefrom or differences in polarization directions due to magnetic Kerr effects are detected so as to obtain a reproduction RF signal. Then, the obtained reproduction RF signal is processed according to a constant threshold value, for example, so as to detect the binary data.
For the purpose of increasing a recording density of information, a method of recording, not binary data, but ternary or further multivalued-level data, has been contrived; however, in an optical disc, a level fluctuation or an amplitude fluctuation occurs in a reproduction signal due to various factors, such as differences in reflectance among various types of optical discs, or differences in reproduction frequency characteristics between inner and outer tracks in one optical disc. Therefore, when a threshold value used in detecting multivalued-level data is constant, the reproduction signal is possibly detected at a wrong value. Especially, since multivalued-level data, such as three-valued or four-valued data, require a plurality of threshold values, the multivalued-level data is more likely to be detected incorrectly.
For the purpose of solving the above-described problems, Japanese Laid-Open Patent Application No. 5-54391 discloses as follows. A reproduction signal reproduced from a recording medium on which multivalued-level data is recorded is subjected to an A/D conversion according to a reference clock, and a signal level data obtained by this A/D conversion is compared with threshold values so as to detect the multivalued-level data; in this course, the signal level data obtained from the reproduction signal by the above-mentioned A/D conversion is stored in a memory means on a predetermined signal unit basis, and distribution information of the signal level data is obtained for each of the predetermined signal units stored in the memory means; then, this distribution information determines threshold values used for detecting the multivalued-level data from the signal level data in the present signal unit. Further, the above-mentioned course is based on a use of a format in which a proportion of level values of recorded data is fixed.
For example, on a predetermined signal unit basis, such as a reproduction signal in sector units in a recording format on an optical disc, a signal level data obtained from the reproduction signal by an A/D conversion is temporarily stored in a memory, and distribution information of occurrence frequencies thereof is obtained by using a CPU so as to obtain optimal threshold values as criteria for detecting multivalued-level data; and multivalued-level data is redetected from the stored signal level data by using the threshold values. Therefore, even when a level fluctuation or an amplitude fluctuation occurs in the reproduction signal, the multivalued-level data is less likely to be detected incorrectly so as to realize an accurate multi-value detection, which consequently promotes an increase in a recording density of recording data.
In Japanese Laid-Open Patent Application No. 5-54391, for the purpose of dealing with a fluctuation in a long period, such as a signal level fluctuation in inner and outer tracks of an optical disc, threshold values used for detecting multivalued-level data are optimized on an individual sector basis, for example. Additionally, a restriction is imposed on a format as a precondition so as to orderly separate multivalued-levels which include variations.
By the way, increasing a recording density of information necessitates, not only the use of multivalued data, but also a miniaturization of a unit of data recording/reproduction (hereinafter referred to as an “information cell” or simply a “cell”). When the information cell is so miniaturized that a plurality of information cells become encompassed within a reproduction light beam spot (see a description made later with reference to FIG. 1A to FIG. 1D), an output level of a reproduction signal of the information cell comes under influences of preceding and ensuing information cells such that output levels of reproduction signals corresponding to a same multivalued level shift (vary) depending on which multivalued level the preceding and ensuing information cells possess, as a result of which a variation (an expansion) of each of multi-valued levels becomes larger, apparently. Therefore, when compiled in a long period, such as a sector unit, distributions of adjacent multi-valued levels overlap one another. FIG. 2, details of which will be described hereinafter, shows an actual example of a level distribution of reproduction signals in a case where a multi-valued level (=eight-valued data level) is recorded in one information cell, and the reproduction light beam spot has a size encompassing three information cells. In this example, distributions overlap between levels 1 and 2, levels 2 and 3, and levels 4 and 5.
When applying a detecting method as disclosed in Japanese Laid-Open Patent Application No. 5-54391 to such a reproduction signal as described above, there is a problem that a level of an information cell is highly likely to be detected incorrectly due to the overlap of distributions, in whatever manner the threshold values are determined. Further, the restriction imposed on a degree of freedom regarding a format is also a problem.